The semiconductor wafer should be cut into separate device chips or dies, which is an indispensable step for manufacturing semiconductor device or integrated circuit, and is one of last fabrication steps. In the past, a large size wafer was mechanically cut into separate dies using diamond knife (blade). However, mechanical cutting is very time-consuming and is apt to disrupt the wafer with very small (thin) thickness. Recently, the mechanical cutting has gradually been replaced by laser cutting for cutting the relatively brittle wafer such as gallium arsenide of III-v group. The semiconductor surface is irradiated by a focused laser light with high power and is decomposed due to the increase of local temperature. The advantages for laser cutting include rapid cutting (only one fifth of time for mechanical cutting) and that the relatively brittle semiconductor wafer being not easily disrupted.
For the gallium arsenide wafer being cut, the main problem in laser cutting is the recast of gallium arsenide residue and the microcracks at the cutting interface. FIG. 1 is a schematic view showing the cross section around the laser cutting groove. During the irradiation process of focused laser light with high power, the arsenic vapor and small particles of gallium arsenide residues would be produced due to the increase of local temperature and decomposition of gallium arsenide. The gallium arsenide residues would recast at cutting edge and on device surface. For avoiding the effect of gallium arsenide residues on the device property, the device surface must be covered with a protection layer and the gallium arsenide residues should be removed by etching method after laser cutting. The selection of materials for protection layer must consider whether the materials can resist the high temperature produced by focused laser light, and whether the materials must have good capability for adhering and covering on wafer surface. At present, the protection layer is usually made by water soluble PVA. However, the water soluble protection layer would be in the same time dissolved during the etching of the gallium arsenide residues. Therefore, the gallium arsenide around the device would also be etched in the process of the etching of gallium arsenide residues and produce the phenomenon of etching undercut at device edge, which would affect seriously the yield and reliability of devices after laser cutting.
A way to resolve the problem stated above is to use a water insoluble material as the protection layer. However, the selection of materials for protection layer must further consider other factors. For example, a tape, such as blue tape or UV tape, is generally used to fix the wafer during the process of laser cutting. Therefore, in addition to the corrosion resistance to the etching solution for gallium arsenide residues, the factor concerning the tape must be further considered. Because a general tape is easily deteriorated at a temperature higher than 80° C., a process performing at a relatively low temperature must be used to cover and remove the protection layer. Furthermore, the tape could also be deteriorated in some acidic and basic solutions, so that any solutions used in fabrication process must ensure the property of tape not being destroyed.
In view of the problems stated above, a suitable fabrication method must be developed to prevent the gallium arsenide wafer from the phenomenon of etching undercut at device edge during the etching process for removing the gallium arsenide residues after laser cutting. This is now an important issue about a fabrication method for dicing of gallium arsenide semiconductor wafer by using laser cutting techniques.